Gate turn-off (GTO) switches are widely known semiconductor devices capable of controlling a load current in response to a control signal. Such devices are often referred to as thyristors. Currently, it is desired to increase the capacity of such devices to handle greater currents and voltages. In doing so, however the turn off characteristics of the device must be maintained to as great a degree as possible. Typically, a GTO switch is a 4-layer vertical structure having a P-type (anode) layer on the bottom followed by N-type and P-type inner (base) layers and an N-type upper (cathode) layer. Recently, N-type shorts have been utilized extending up through the anode layer to the N-type base layer. The P-type base layer is brought to the upper surface of the device at some point to allow contact. This contact is commonly referred to as the gate. The upper two layers of the device have most commonly been manufactured by means of a double diffused process flow. That is, the P-type base is first defined by diffusion of a P-type dopant and then the N-type cathode is defined by the diffusion of an N-type dopant into the previously diffused P-type region.
In order to control the turn-off characteristics of a GTO switch it is necessary to carefully control the sheet resistance of the P-type base region. It has been found that this is relatively difficult to do using a double diffused process. This problem has become increasingly severe as the current and voltage capacities of the device are increased. A further requirement for GTO switches is the capacity to withstand a relatively high reverse gate voltage. The prior art double diffused method of forming the gate and cathode regions is not reliably capable of producing a reverse gate voltage capacity significantly in excess of 15 volts.